In today's technological world, there is an increasing need for membrane switch assemblies that can be mounted on cathode ray tubes (CRT's). An ever increasing amount of software is becoming available that requires the user to answer questions by pressing appropriate areas of a CRT equipped with a membrane switch assembly.
These assemblies are generally made from two layers of essentially transparent film, the internal surfaces of each layer having conductors deposited thereon, separated by a third layer of film containing openings for switch sites. The conductors on the layers cross and are normally spaced from each other at the switch sites.
When such a device is operated, it is highly desirable that the assembly be essentially optically clear to the operator. For this reason, assemblies having conductive ink traces are generally unacceptable. The width of the ink trace is visible to the naked eye. Assemblies of the type having narrow hairline conductors are generally more acceptable.
Use of the hairline conductors, however, can cause other problems. An electrical circuit can be completed only when the conductors from the two layers are brought in contact with each other. The conductors, being extremely narrow, present a relatively small switch site area. Failure to press on the point at which the conductors cross usually means the circuit is not completed. Furthermore, a break in a hairline conductor also prevents the completion of the circuit.
By using essentially parallel double hairline conductors instead of single hairline conductors, the invention disclosed herein solves the above problems by providing a larger area for applying pressure to complete the circuit and also by providing for redundancy in the event a break should occur in a hairline conductor. At each switch site there are four possible points of electrical contact. In addition to the double hairline conductor, the disclosed invention uses a polymer spacer that is deposited on both layers of film over the hairline conductors themselves, except for the switch sites. The polymer therefore is a plating mask as well as a spacer.
Plating of the conductors in membrane switches of this type is often desirable. It is more economical to plate just the switch sites rather than the complete conductors. Having a spacer that also serves as a plating mask eliminates the need to apply and then remove a separate plating mask from the conductors.